224 MOLLUSCA. 



valve has a notch under the anterior ear; and the hinge has 

 one or two cardinal teeth, sometimes with an elongated pos- 

 terior tooth. The true Aviculcs are represented by very numer- 

 ous fossil species, extending from the Lower Silurian Rocks to 

 the present day. Of the sub-genera of Avicula, the three most 

 important fossil forms are Ambonychia, Pteritiea, and Cardiola, 

 the place of the last in this family being somewhat doubtful. 

 In the Ambonychia (fig. 177) the valves are gibbous, nearly 

 equal, and the anterior ear is almost obsolete. They extend 

 from the Lower Silurian to the Carboniferous. The Pterineas 

 (fig. 178) are very abundant in the Silurian Rocks, especially 

 in the upper division of the series, and they extend to the Car- 



Fig. 178. A, Pterineasub-falcata; B, Cardiola interrupta; C, Cardiolafibrosa. 

 (After M'Coy and Salter.) Silurian. 



boniferous Rocks. They have a shell with large ears and very 

 oblique, the hinge-area being long and straight. The Cardiolce 

 have an oblique, equivalve shell (fig. 178, B), radiately ribbed, 

 with prominent beaks and a short hinge-area. They are mainly 

 characteristic of the Upper Silurian Rocks, but they have been 

 stated to occur in the Lower Silurian, and they are found in 

 the Devonian. 



The shells of the genus Posidonomya are very thin, con- 

 centrically striated, equivalve and earless. They extend from 

 the Silurian to the Trias, but are especially characteristic of 

 the Carboniferous Rocks. Many of the smaller shells referred 

 to Posidonomya are undoubtedly referable to the Crustacean 

 genus Estheria, 



The genus Gcn>illia comprises a number of fossil shells, 

 which range from the Carboniferous Rocks to the Chalk, and 

 which are very like the true Avicula. The shell is elongated, 

 the anterior ear small, and the posterior ear broad and wing- 

 like. Nearly allied to Gervillia is the genus Perna, which 

 commenced in the Trias, and is represented in recent seas. 



Nearly related to both Gervillia and Perna is the genus 

 Inoceramus (figs. 179, 180), which is entirely confined to the 

 Secondary period, and is mainly characteristic of the Creta- 



